Modern residential construction commonly utilizes insulation materials that are light weight with high thermal resistance (R-values) and thermal transmittance (U-factors) but do not take into account the advantageous uses of heavier, dense thermal mass such as concrete. Thermal mass in the most general sense is any material that has the capacity to store thermal energy or heat. When used correctly, thermal mass can significantly delay the requirements for electric heating and air conditioning systems in a structure or residential dwelling to a different time periods, such as when electric utility rates are lower.
In much of the USA, On Peak and Off Peak hours are determined by the local electric utility and typically are different for summer and winter months. In summer months On Peak hours are typically from 1:00 pm-7:00 pm Monday through Friday. In winter months On Peak hours are typically 7:00 am-12:00 noon Monday through Friday. At all other times, or Off Peak, electrical utility rates are as much as 70% lower than On Peak rates. Shifting the time at which a home owner is using electricity, results in substantial electric utility cost savings to the home owner. Additionally, shifting peak electrical loads can reduce the number of power plants required, since power plants are designed to provide power at peak loads, and thus reduce the resultant pollution of the atmosphere.
With buildings, heat flow is referred to in a number of different ways. The most common reference is “R-value,” or resistance to heat flow. The higher the R-value of a material, the better it is at resisting heat loss or heat gain. U-factor, or “U-value” as it is often called, is a measure of the flow of heat-thermal transmittance through a material, given a difference in temperature on either side. The U-factor is the number of British Thermal Units (“BTU”) of energy passing through a square foot of the material in an hour for every degree Fahrenheit difference in temperature across the material (Btu/ft2hr° F.). Materials that are very good at resisting the flow of heat (high R-value, low U-factor) can serve as good insulation materials. The R-value and U-factor are the inverse of one another: U=1/ R.
Materials have another property called heat capacity that can affect their thermal energy performance in certain situations. Heat capacity is a measure of how much heat a material can hold. The property is most significant with heavy, high-thermal-mass materials such as concrete. Concrete is an ideal material for thermal mass as this material has high specific heat, high density, and low thermal conductivity.
Thermal Mass represents and reflects the ability of a material, or a combination of materials, to store thermal energy. This property is characterized by the mass of the material and its specific heat. The thermal mass is described with the following equation:
  Q  =            c      ·      m        ⁢          dT      dt                      Where        Q=Heat flow        c=Specific heat of mass material        m=Mass        T=Temperature        t=Time        
Materials with low thermal conductivity are able to slowly gather and store heat, and then slowly release heat. Materials with high thermal mass can gather and release relatively large quantities of heat per unit volume compared to other materials.
Thermal mass should not be confused with insulation. Materials used for insulation typically have much lower thermal conductivity than materials used for thermal mass and generally do not have a high capacity to store heat. They can reduce unwanted heat transfer but are not significant sources of heat, or heat storage, in themselves.
In the typical and historical application of thermal mass in a residential dwelling, the walls and floors are constructed of concrete. The concrete is heated by the sun during the day, which stores the heat and releases it at night for heating the interior structure. Conversely, as the concrete floors and walls cool down at night, they help to keep the structure cool the next day. This approach to thermal mass, or thermal mass effect, has limitations that have restricted its use. For example, high-mass walls can significantly outperform low-mass walls of comparable steady-state R-value, i.e. they can achieve a higher “mass-enhanced R-value.” However, this mass-enhanced R-value is only significant when the daily outdoor temperatures cycle above and below indoor temperatures within a 24-hour period. Thus, high-mass walls and floors are most beneficial to moderate climates that have high daily temperature swings above and below the desired indoor temperature setpoint, such as in areas like New Mexico.
Therefore, there is a need to design heating and cooling systems that can more efficiently utilize thermal mass and geothermal energy as an alternate heating and cooling source for a dwelling, such as a commercial or residential structure.